This invention relates to a synthetic resinous powder product to be laser-sintered in a selective laser sintering machine, such as a SINTERSTATION 2000 system manufactured and sold by DTM Corporation. The laser-sinterable powder (referred to as "sinterable powder" herein) is "designed" or "tailored" to incorporate specific physical properties uniquely adapted to form a bed (of powder) upon which a sintering laser in the infra-red region is directed.
Prior art sinterablc powders are unable to yield a sintered part which, for most purposes, appears to be a duplicate of one which is isotropically molded. Moreover, conventional sinterable powders form a bed which generally lacks the ability to provide the exigent heat transfer characteristics which determine whether a sintered part will be distorted, even if it is successfully completed. Since a layer of particles typically rolled out of the feed bed and onto the part bed of a selective laser sintering machine, is about 8 mils (200 .mu.m) such powders used had a maximum particle diameter which was less than 200 .mu.m and whatever "fines" were generated in the course of grinding the powder to the desired mesh size, irrespective of the distribution of particle sizes in the powder.
It has been observed that the selective laser sintering of amorphous polymer powders typically results in finished parts that are somewhat porous. Typical amorphous polymers exhibit a second order thermal transition at a temperature that is commonly referred to as the "glass transition" temperature, and also exhibit a gradual decrease in viscosity when heated above this temperature. In the selective laser sintering of amorphous polymers, the part bed is maintained at a temperature near the glass transition temperature, with the powder being heated by the laser at the part locations to a temperature beyond the glass transition temperature to produce useful parts, since viscosity controls the kinetics of densification. While it may be at least theoretically possible to build fully dense (i.e., non-porous) parts from amorphous polymers, practical considerations arising from the use of high power lasers, such as thermal control, material degradation, and growth (undesired sintering of powder outside of the scanned regions) have prevented the production of such fully dense parts. Further, it has been observed that the selective laser sintering of amorphous polymer powders is also vulnerable to "in-build curl", where subsequent sintered layers added to the part shrink onto the solid substrate, causing the part to warp out of the part bed.
The sinterable powders of the present invention are directed to yielding a sintered article ("part") which, though porous, not only has the precise dimensions of the part desired, but also is so nearly fully dense (hence referred to as "near-fully dense") as to mimic the flexural modulus and maximum stress at yield (psi), of the article, had it been fully dense, for example, if it had been isotropically molded.
In addition, the properties deliberately inculcated in the sinterable powder are unexpectedly effective to provide the bed with sufficient porosity to permit cooling gas to be flowed downwardly through it, yet maintaining a quiescent bed in which the sintered part mimics the properties of a molded article.
The term "near-fully dense" refers to a slightly porous article which has a density in the range from 80%-95% (void fraction from 0.2 to as low as 0.05), typically from 85%-90% of the density (void fraction 0.15-0.1) of a compression molded article which is deemed to be fully dense.
The term "fully dense" refers to an article having essentially no measurable porosity, as is the case when an article of a synthetic resinous powder is compression (or injection) molded from a homogeneous mass of fluent polymer in which mass individual particles have lost their identity.
By a "quiescent bed" we refer to one upon the surface of which the particles are not active, that is, do not move sufficiently to affect the sintering of each layer spread upon a preceding slice sintered in the part bed. The bed is not disrupted by the downward flow of gas, so that the bed appears to be static.
To date, despite great efforts having been focussed on a hunt for the formulation of a sinterable powder which will yield a near-fully dense part, that formulation has successfully eluded the hunt. The goal is therefore to produce a mass of primary particles of a synthetic resin which has properties specifically tailored to be delivered by a roller to the "part bed" of a selective laser sintering machine, then sintered into a near-fully dense prototype of a fully dense article.
A powder dispenser system deposits a "layer" of powder from a "powder feed bed" or "feed bed" into a "par bed" which is the target area. The term "layer" is used herein to refer to a predetermined depth (or thickness) of powder deposited in the part bed before it is sintered.
The term "prototype" refers to an article which has essentially the same dimensions of a compression or injection molded article of the same material. The porous prototype is visually essentially indistinguishable from the molded article, and functions in essentially the same manner as the molded article which is non-porous or fully dense. The flexural modulus, flexural strength and flexural elongation at yield, are essentially indistinguishable from the values obtained for a molded article. One is distinguishable from the other only because the prototype has a substantially lower, typically less than one-half, the ultimate tensile elongation (%), and notched Izod impact (ft-lb/in), than a compression molded article, though the prototype's tensile modulus, tensile strength, and elongation at yield are substantially the same as those of the compression molded article (see Table 1 hereinbelow). In Table 1, the values given in square brackets are the standard deviations under the particular conditions under which the measurements were made.
The tensile elongation, ultimate (%), and notched Izod impact are lower for the prototype because of its slight porosity. Therefore the energy to break, which is the area under the stress curve up to the point of break at ultimate elongation, is also very much lower than that for the compression molded article. As is well known, any small imperfections in a homogeneous article will be reflected in the ultimate tensile elongation and notched Izod impact. However, confirmation that the molded article has been closely replicated is obtained by a comparison of the fracture surfaces of the prototype and of the molded article. Photomicrographs show that these fracture surfaces of the prototype are visually essentially indistinguishable from fracture surfaces of an isotropically molded non-porous part except for the presence of a profusion of cavities having an average diameter in the range from 1 .mu.m-30 .mu.m randomly scattered throughout said part, indicating similar creep and fatigue characteristics. As one would expect, the cavities provide evidence of the porosity of the prototype. Therefore it is fair to state that, except for the lower ultimate elongation or Izod impact of the prototype, due to its slight porosity, the prototype fails in the same manner as the molded article.
A laser control mechanism operates to direct and move the laser beam and to modulate it, so as to sinter only the powder disposed within defined boundaries (hence "selectively sintered"), to produce a desired "slice" of the part. The term "slice" is used herein to refer to a sintered portion of a deposited layer of powder. The control mechanism operates selectively to sinter sequential layers of powder, producing a completed part comprising a plurality of slices sintered together. The defined boundaries of each slice corresponds to respective cross-sectional regions of the part. Preferably, the control mechanism includes a computer--e.g. a CAD/CAM system to determine the defined boundaries for each slice. That is, given the overall dimensions and configuration of the part, the computer determines the defined boundaries for each slice and operates the laser control mechanism in accordance with the defined boundaries for each slice. Alternatively, the computer can be initially programmed with the defined boundaries for each slice.
The part is produced by depositing a first portion of sinterable powder onto a target surface of the part bed, scanning the directed laser over the target surface, and sintering a first layer of the first portion of powder on the target surface to form the first slice. The powder is thus sintered by operating the directed laser beam within the boundaries defining the first slice, with high enough energy (termed "fluence") to sinter the powder. The first slice corresponds to a first cross-sectional region of the part.
A second portion of powder is deposited onto the surface of the part bed and that of the first sintered slice lying thereon, and the directed laser beam scanned over the powder overlying the first sintered slice. A second layer of the second portion of powder is thus sintered by operating the laser beam within the boundaries which then define the second slice. The second sintered slice is formed at high enough a temperature that it is sintered to the first slice, the two slices becoming a cohesive mass. Successive layers of powder are deposited onto the previously sintered slices, each layer being sintered in turn to form a slice.
Repetition of the foregoing steps results in the formation of a laser-sintered article lying in a "part bed" of powder which continually presents the target surface. If the particles of powder at the boundaries of each layer are overheated sufficiently to be melted, unmelted particles immediately outside the boundaries adhere to the molten particles within, and the desired sharp definition of the surface of the sintered article is lost. Without sharp definition at the boundaries, the article cannot be used as a prototype.
Particles of powder adjacent the surfaces of the article to be formed should resist being strongly adhered to those surfaces. When particles are not so strongly adhered they are referred to as "fuzz" because fuzz is easily dislodged from the surface, manually, and the dislodged particles retain most of their individual identities. Particles so tightly adhered to the surface as to be removed satisfactorily only with a machining step, are referred to as "growth". Such growth makes a sintered part unfit for the purpose at hand, namely to function as a prototype for a compression molded part.
A method for sintering a powder into a shaped article in a selective laser sintering machine is disclosed in U.S. Pat. Nos. 4,247,508 to Housholder; 4,863,538 and 5,132,143 to Deckard; 4,938,816 to Beaman et al.; and, 4,944,817 to Bourell et al., the relevant disclosure of each of which is incorporated by reference thereto as if fully set forth herein. "Sintering" is defined as the heating of the powder to a temperature which causes viscous flow only at contiguous boundaries of its particles, with at least some portion of substantially all particles remaining solid. Such sintering causes coalescence of particles into a sintered solid mass the bulk density of which is increased compared to the bulk density of the powder particles before they were sintered; and, a part formed by "slice-wise" joining of plural vertically contiguous layers which are sintered into stacked "slices" is therefore said to be autogenously densified. A layer of powder is confined by vertically spaced apart horizontal planes, no more than about 250 .mu.m apart and each slice is typically in the range from 50 .mu.m to 180 .mu.m thick.
A specific goal of this invention is to produce a sinterable powder of a single, that is, unblended, synthetic resin the molecular weight range and molecular weight distribution of which may be controlled to produce a powder which, when exposed to the laser beam, is heated so that the outer portions of each particle have a narrowly defined range of viscosity which results in the fusion of successive slices.
It must be remembered that before the powder can be sintered in the part bed, it must be delivered from the feed bed to the part bed upon which the powder is distributed in a thin, even layer about 125.mu. thick, by the roller of the selective laser sintering machine. Each distributed layer should be thin and evenly distributed because the temperature gradient through the cross-section of the sintered slice must be small, typically &lt;5.degree. C., more preferably &lt;2.degree. C., and most preferably &lt;1.degree. C. To meet this demanding criterion, the powder must be freely flowable from the feed bed onto the part bed.
By "freely flowable" we refer to a mass of small particles, the major portion of which, and preferable all of which have a sphericity of at least 0.5, and preferably from 0.7 to 0.9 or higher, so that the mass tends to flow steadily and consistently as individual particles. Though such flow is conventionally considered a characteristic of a powder which flows through an orifice slightly larger than the largest particle, such flow (through an orifice) is of less importance than the ability of the powder to be picked up in the nip of a rotating roller and transported by it as an elongated fluent mass of individual particles urged along by the roller. A freely flowable powder has the property of being able to be urged as a dynamic elongated mass, referred to as a "rolling bank" of powder, by the rotating roller, even at a temperature near T.sub.s the "softening point" of the powder.
At T.sub.s, the powder is on the verge of not being flowingly transportable as a rolling bank against a rotating roller. By "softening point" we refer to T.sub.s, at which a powder's storage modulus (G'.sub.s) has decreased substantially from its value of G' at room temperature. At or above T.sub.s the storage modulus G'.sub.s of a sintered slice of the powder is low enough so as not to let it "curl". By "curl" we refer to the slice becoming non-planar, one or more portions or corners of the slice rising more than about 50 .mu.m above the surface of the last (uppermost) slice in the horizontal x-y-plane.
A slice will curl when there is a too-large mismatch between the temperature of the initial slice sintered by the laser and the bed of powder on which it lies; or, between powder freshly spread over a just-sintered slice and the temperature at the upper interface of the slice and the freshly spread powder. Such a mismatch is the result of "differential heating". The importance of countering curl is most critical when the first slice is formed. If the first slice curls, the roller spreading the next layer of powder over the slice will push the slice off the surface of the part bed.
If the powder is transported from the feed bed to the part bed in which a hot slice is embedded, and the temperature at the interface T.sub.i between the hot upper surface of the slice and the freshly spread powder is high enough to raise the temperature of the freshly spread powder above T.sub.s, this powder cannot be rollingly distributed over the hot slice because the powder sticks and smears over the hot slice. The indication is that the slice is too hot.
If the powder in the feed bed is too cool, that is, so cool that the equilibrium temperature on the surface of the hot, embedded slice is such that the temperature of the freshly spread powder is below T.sub.s, the slice will curl.
The slice will not curl when the powder spread over it reaches an equilibrium temperature at the interface, and the equilibrium temperature is at or above T.sub.s. the precise temperature T.sub.i at the interface is difficult to measure, but to form successive slices cohesively sintered together, the temperature of the powder at the interface must be above T.sub.s, but below the powder's "sticky point" or "caking temperature" T.sub.c at which the powder itself will not flow.
By "sticky" we infer that the force required to separate contiguous particles has exceeded an acceptable limit for the purpose at hand. This caking temperature T.sub.c may be considered to be reached when a critical storage modulus (G'.sub.c) of the powder has been reached or exceeded. The storage modulus is a property of the powder akin to a material's tensile strength and can be measured directly with a Rheometrics dynamic mechanical analyzer.
To form a sintered part in a selective laser sintering machine, an initial slice is sintered from powder held in the part bed at near T.sub.s but well below T.sub.c. By "near T.sub.s " we refer to a temperature within about 5.degree. C. of T.sub.s, that is T.sub.s .+-.5, preferably T.sub.s .+-.2.
Immediately after the initial slice is formed, the slice is much hotter than the powder on which it rests. Therefore a relatively cool powder, as much as about 40.degree. C., but more typically about 20.degree. C. below its T.sub.s, may be spread over the hot slice and the interface temperature raises the temperature of the powder to near T.sub.s. As the powder is spread evenly over the hot slice it is to remain cool enough to be spread, but soon thereafter, due to heat transfer at the interface, must reach or exceed T.sub.s, or the just-sintered slice will curl; that is, the temperature of the powder preferably enters the "window of sinterability". This window may be measured by running two DSC (differential scanning calorimetry) curves on the same sample of powder, sequentially, with a minimum of delay between the two runs, one run heating the sample past its melting point, the other run, cooling the sample from above its melting point until it recrystallizes. The difference between the onset of melting in the heating curve, Tm, and the onset of supercooling in the cooling curve, Tsc, is a measure of the width of the window of sinterability (see FIG. 6).
To ensure that the powder from the feed bed will form a rolling bank even when it is rolled across the hot slice, the powder is usually stored in the feed bed at a storage temperature in the range from 2.degree. C. to 40.degree. C. below the powder's T.sub.s and transferred at this storage temperature to the part bed, the feed bed temperature depending upon how quickly a layer of powder spread over a just-sintered slice enters the window of sinterability. The T.sub.s may be visually easily obtained--when the powder is too hot to form a rolling bank, it has reached or exceeded its T.sub.s.
It will now be realized that the cooler the powder (below T.sub.s) the higher the risk of curling, if the interface temperature is not high enough to raise the temperature of the layer of powder at least to T.sub.s. A commensurate risk accrues with a powder stored at too high a temperature. The storage temperature is too high, though the powder forms a rolling bank, when the powder smears or sticks as it traverses the slice, an indication that the powder overlying the slice has not only exceeded T.sub.s but also reached (or gone beyond) T.sub.c.
Thus, though it is difficult to measure the interface temperature, or to measure T.sub.c with a temperature probe, so as to measure the width of the window, it can be done visually. When the rolling bank of powder sticks or smears over the last-sintered slice, the T.sub.c of the powder has been reached or exceeded. Thus with visual evidence once can determine the temperature range (T.sub.c -T.sub.s) which is the window of sinterability or the "selective laser sintering operating window", so referred to because the powder cannot be sintered successfully at a temperature outside this selective-laser-sintering-window. (see FIG. 6).
At the start of a sintering cycle it is best to maintain the temperature of the upper layer of the part bed at T.sub.s, preferably 0.5.degree.-2.degree. C. above T.sub.s so that the uppermost layer is presented to the laser beam in the selective-laser-sintering-window. After the first slice is formed, feed is rolled out from the feed bed at as high a temperature as will permit a rolling bank of powder to be transferred to the part bed. The most desirable powders are freely flowable in a rolling bank at a temperature only about 5.degree. C. below their T.sub.s.
However, as the mass of the sintered slices accumulates in the part bed, the sintered mass provides a large heat sink which transfers heat to each layer of powder freshly spread over the hot mass, thus allowing a relatively cool powder, as much as 30.degree. C., more typically 20.degree. C., lower than T.sub.s to be transferred from the feed bed, yet quickly come to equilibrium in the selective-laser-sintering-window as the layer is spread over the last preceding slice. Thus, when each layer is sintered, the later-formed slices will not curl.
It is important that the powder be "freely flowable" from the feed bed, preferably at a temperature sufficiently near T.sub.s to ensure that the last-sintered slice will not curl when the powder is spread upon it. As already pointed out above, if the first slice formed curls, no further progress can be made. A fresh start must be made to sinter the part.
A powder is not freely flowable when the temperature at which it is held or distributed exceeds its softening point. The powder cakes and does not flow at all when the caking temperature is reached. For example, one may consider that at T.sub.c, G'.sub.s decreases to a critical G'.sub.c, in which case the caking temperature T.sub.c may also be referred to as the "G'.sub.c temperature".
It is possible to transfer powder from the feed bed to the part bed at above T.sub.s if the impaired flowability allows one to do so, and the risk of operating too close to T.sub.c is acceptable. Generally a powder does not form a rolling bank at or above its T.sub.s.
According to one aspect of the invention, it is preferred that the powder used in the selective laser sintering process be sinterable in a wide selective-laser-sintering-window. Though within narrow limits, the `width` (in .degree.C.) of the window, varies from the start of the cycle and at the end (particularly when a large part is formed, as explained above). The width of the window also varies depending upon the composition of the powder. This width ranges from about 2.degree. C. to about 25.degree. C.; more typically, it is about 5.degree. C.-15.degree. C. With a powder which is freely flowable over a wide temperature range, one is able to form, in the best mode, a solid, near-fully dense article when the powder is sintered in a selective laser sintering machine which uses a roller to spread the powder.
The temperature at which G'.sub.s is measured is believed to not be critical, provided the G'.sub.c temperature offers an adequately large selective-laser-sintering-window. Most desirable laser-sinterable powders have an unexpectedly common characteristic, namely that the value of their G'.sub.c is narrowly defined in the range from 1.times.10.sup.6 dynes/cm.sup.2 to 3.times.10.sup.6 dynes/cm.sub.2.
For a crystalline powder (100% crystallinity), the softening point is its melting point Tm. Therefore G'.sub.s and G'.sub.c are essentially identical and there is no G'-window. For an amorphous powder, its softening point is its initial glass transition temperature Tg. An amorphous powder can offer a large window of sinterability but because its viscosity decreases too slowly as temperature increases and the G'.sub.-c limit of the selective-laser-sintering-window is approached, the viscosity is still too high. That is, the viscosity is too high to allow requisite interchain diffusion at the boundaries of the particles without melting the entire particle. Therefore an amorphous powder is difficult to sinter to near-full density, so that powders which qualify as the product of this invention are semi-crystalline powders such as nylon, polybutylene terephthalate (PBT) and polyacetal (PA) which provide signs of crystalline order under X-ray examination, and show a crystalline melting point Tm as well as a glass transition temperature Tg. Because the crystallinity is largely controlled by the number and distribution of branches along the chain, the crystallinity varies, bulky side chains or very long chains each resulting in a reduction of the rate of crystallization. Preferred polymers have a crystallinity in the range from 10%-90%, more preferably from 15%-60%.
To summarize, the selective laser sintering process is used to make 3-D objects, layer-upon-layer sequentially and in an additive manner. The process is more fully described in the '538 Deckard patent and comprises the following steps:
(1) Powder from the feed bed is "rolled out" by a roller, to a part bed where the powder is deposited and leveled into a thin layer, typically about 125 .mu.m (0.005") in depth. PA1 (2) Following a pattern obtained from a two dimensional (2-D) section of a 3-D CAD model, a CO2 laser "sinters" the thin layer in the target region of the part bed and generates a first slice of sintered powder in a two-dimensional ("2-D") shape. Directions for the pattern, and each subsequent pattern for successive slices corresponding to a desired three-dimensional ("3-D") prototype are stored in a computer-controller. It is critical for a slice-upon-slice construction of the prototype that the laminar, planar shape of each slice of sintered powder be maintained, that is, "without curling". PA1 (3) A second layer of powder from the feed bed is then deposited and leveled over the just-sintered layer in the part bed, forming a second slice sintered to the first slice. PA1 (4) The computer-controller makes incremental progress to the next 2-D section, the geometry of which is provided from the 3-D model, and instructs the laser/scanner system to sinter the regions of the bed desired for successive 2-D sections. PA1 (5) Still another layer of powder is deposited from the feed bed and leveled over the just-sintered layer in the part bed. PA1 (6) The foregoing steps are repeated, seriatim, until all layers have been deposited and sequentially sintered into slices corresponding to successive sections of the 3-D model. PA1 (7) The sintered 3-D object is thus embedded in the part bed, supported by unsintered powder, and the sintered part can be removed once the bed has cooled. PA1 (8) Any powder that adheres to the 3-D prototype's surface as "fuzz" is then mechanically removed. PA1 (9) The surfaces of the 3-D prototype may be finished to provide an appropriate surface for a predetermined use. PA1 (a) a major portion by weight of the powder, and preferably essentially all the powder having a sphericity in the range from greater than 0.5 to 0.9, and a two-tier particle size distribution of primary particles having an average diameter smaller from than 180 .mu.m, with substantially no particles &gt;180 .mu.m, provided further that the number average ratio of particles smaller than 53 .mu.m is greater than 80%, preferably greater than 90%, and most preferably greater than 95%, the remaining particles being in the size range from 53 .mu.m to 180 .mu.m; a layer of the powder no more than 250 .mu.m deep absorbs essentially all infra-red energy at the 10.6 .mu.m wavelength beamed therethrough, and absorbs more than 50% of that energy in a layer no more than 180 .mu.m thick; PA1 (b) a crystallinity in the range from 10% to 90%, preferably from 15% to 60%, a number average molecular weight in the range from about 30,000 to 500,000, preferably 60,000-300,000, and a molecular weight distribution in the range from 1 to 5, preferably from 1.2 to 3; and, PA1 (c) a "selective-laser-sintering-window" in the temperature range from 2.degree. C.-25.degree. C. between the softening temperature T.sub.s of the powder and its "caking temperature" T.sub.c, such that the powder has a "flow time" of &lt;20 sec for 100 g in a funnel test (ASTM D1895-61T) at a temperature near T.sub.s in a range from 70.degree. C. to 220.degree. C., but below the powder's T.sub.c ; and, PA1 (d) a melt viscosity in the range from 100-10.sup.4 poise (10-1000 Pa-sec) when the temperature of the powder being sintered exceeds T.sub.c in less time than is required to melt contiguous large particles &gt;53 .mu.m. PA1 (i) a "selective-laser-sintering-window" in the range from T.sub.s to T.sub.c ; and, PA1 (ii) a `part bed` in which the sintered part is removably embedded while it dissipates heat to generate a temperature profile defined by sequential positive and negative temperature gradients, in a vertical plane through the part bed; such a gradient occurs when the uppermost slice is less than 250 .mu.m thick, and is near T.sub.s of the powder, and the temperature of the sintered part is near T.sub.c. Further, the gradient from the upper surface of the part bed to the maximum temperature in the horizontal zone in which the sintered part lies, is positive, the temperature increasing at a rate in the range from 0.2.degree. C./cm (0.5.degree. C./in) to 2.degree. C./cm (5.degree. C./in) of vertical depth; and, from the maximum temperature in the horizontal zone, to the bottom of the bed, the gradient is negative, the temperature decreasing at a rate in the range from 0.2.degree. C./cm (0.5.degree. C./in) to 2.degree. C./cm (5.degree. C./in).
This invention relates mainly to producing and using a powder which is designed to satisfy the requirements of the first three steps of the process.
Although we have experimentally processed many synthetic resinous powders in the selective laser sintering machine, we have found that few make near-fully dense parts. In most cases the measured values of flexural modulus and maximum stress at yield are at least 30% lower than values obtained made by injection or compression molding the same part. We now understand, and have set forth below, what properties are required of a powder which can be successfully sintered in a selective laser sintering machine, and have accepted, at least for the time being, the many disappointing results we obtained with amorphous polymers such as polycarbonate (PC) and acrylonitrile-butadiene-styrene resins (ABS).
It has now become evident that a semi-crystalline or substantially crystalline organic polymer is the powder of choice if it is to provide the high definition of surface ("lack of growth") which a prototype made from the tailored powder of this invention, provides.
By a "semi-crystalline polymer" or "substantially crystalline polymer" is meant a resin which has at least 10% crystallinity as measured by DSC, preferably from about 15%-90%, and most preferably from about 15-60% crystallinity.
U.S. Pat. No. 5,185,108, issued Feb. 9, 1993, incorporated herein by this reference, teaches that to produce a sintered article of wax having a void fraction (porosity) of 0.1, a two-tier weight distribution of wax particles was necessary. The desired two-tier distribution was produced by a process which directly generated a mass of wax microspheres such that more than half (&gt;50%) the cumulative weight percent is attributable to particles having a diameter greater than a predetermined diameter (100 .mu.m is most preferred for the task now at hand) for the particular purpose of packing at least some, and preferably a major portion of the interstitial spaces between larger particles, with smaller ones.
The two-tier distribution described in U.S. Pat. No. 5,185,108 was arrived at by recognizing that the densest packing of uniform spheres produces a void fraction (porosity) of 0.26 and a packing fraction of 0.74 as illustrated in FIG. 1; and further, by recognizing that the packing factor may be increased by introducing smaller particles into the pore spaces among the larger spheres. As will be evident, the logical conclusion is that the smaller the particles in the pore spaces, the denser will the packed powder (as illustrated in FIG. 2), and the denser will be the pan sintered from the powder.
As will further be evident, the greater the number of small particles relative to the large, in any two-tier distribution, the denser will be the part. Since the goal is to provide a near-fully dense part, logic dictates that one use all small particles, and that they be as small as can be.
However, a mass of such uniformly small particles is not freely flowable. To make it freely flowable one must incorporate larger particles into the mass, much in the same manner as grains of rice are commonly interspersed in finely ground table salt in a salt shaker. Therefore, the tailored powder is a mixture of relatively very large and relatively very small particles in a desirable two-tier particle size distribution for the most desirable sinterable powders.
The demarcation of size in the two-tier distribution and the ratio of the number of small particles to the number of large particles, set forth hereinbelow, are both dictated by the requirements of the selective laser sintering machine.
Further it was found that the rate of heat transfer into the mass of a small particle is so much higher than that into the mass of a large particle, that one could not know either just how large the particles in the upper tier should be, nor how many of such large particles could be present. If the heat transfer to small particles in the bed adjacent the boundaries of each layer was too high, unacceptable growth is generated. If the heat transfer is not high enough, the large particles, namely those &gt;53 .mu.m, in the layer are not sintered, thus forming a defective slice. It is because essentially all these large particles are sintered without being melted, and a substantial number of the small particles &lt;53 .mu.m are melted sufficiently to flow into and fill the interstices between sintered large particles, that the finished sintered part is near-fully dense. Under successful sintering conditions to form a near-fully dense pan, the temperature of the powder must exceed T.sub.s in less time than is required to melt the large particles &gt;53 .mu.m. If the time is too long, large particles will melt and there will be growth on the surfaces of the part; if the time is too short, all the large particles are not sintered. Thus the large particles not only help form a rolling bank, but also fill an important role to maintain the desired transient heat transfer conditions.
It has been found that only a substantially crystalline powder which does not melt sharply, lends itself to the purpose at hand, and only when the powder is stripped of substantially all too-large particles (termed "rocks") larger than 180 .mu.m (80 mesh, U.S. Standard Sieve Series). By "substantially all" we mean that at least 95% of the number of "rocks" in the powder are removed.
It has further been found that a laser-sinterable powder in the proper size range of from about 1 .mu.m-180 .mu.m, may, according to one aspect of the invention, be specified by (i) narrowly defined particle size range and size in a two-tier distribution, and, (ii) the "selective-laser-sintering-window".
According to another aspect of the invention to be described in detail hereinbelow, it has now been realized that the two-tiered particle size distribution is not absolutely necessary in order to create a distortion-free fully dense part in the selective laser sintering process, provided that the recrystallization rate of the material is sufficiently low.
Referring to the first aspect of the invention noted above, the unexpected effect of using the tailored powder with a defined selective-laser-sintering-window is supported by evidence of the sinterability of the powder in this window. The selective-laser-sintering-window is directly correlatable to the powder's fundamental properties defined by its G'.sub.c temperature.
More surprising is that, despite the much larger number of small particles than large in the part bed, it is possible to flow the stream of cooling gas (nitrogen) downwardly through the quiescent bed at low enough a pressure so as not to disturb the particles on and near the surface of the bed sufficiently to cause movement noticeable by the naked eye (hence referred to as "quiescent"). One would expect the pressure drop through a bed of very fine particles, more than 80% of which are smaller than 53 .mu.m (270 mesh) to be relatively high. But the presence of the large particles, coupled with the fact that the powder is delivered from the feed bed and distributed evenly by a roller, rather than being pressed onto the bed, unpredictably provides the requisite porosity in the range from 0.4 to 0.55 to allow through-flow of a gas at superatmospheric pressure in the range from 103 kPa (0.5 psig) to 120 kPa (3 psig), preferably from 107-115 kPa (1-2 psig) with a pressure drop in the range from 3-12 kPa, typically 5-7 kPa, without disturbing a quiescent part bed 30 cm deep.
The part bed formed by the tailored powder is unique not only because its specific use is to generate laser-sintered parts, but because the bed's narrowly defined porosity and defined particle size provides "coolability". In operation, the powder in the part bed is heated by a multiplicity of hot sintered slices to so high a temperature that the powder would reach its caking temperature T.sub.c if the hot bed could not be cooled.
An identifying characteristic of a preheated `part bed` of powder having a two-tiered distribution, with primary particles in the proper size range, stripped of rocks &gt;180 .mu.m, is that the bed is not too tightly packed to permit the flow of cooling gas through the bed. This characteristic allows the part bed to be maintained, during operation sintering a part, with a specified temperature profile which allows formation of a distortion-free sintered part as it is formed slice-wise; and also, after the sintered part is formed, and the part lies in the heated part bed. By "distortion-free" is meant that no linear dimension of the part is out of spec more than .+-.250 .mu.m, and no surface is out of plane by more than .+-.250 .mu.m (20 mils).
Though the importance of a two-tier particle size weight distribution was disclosed with respect specifically to wax particles in U.S. Pat. No. 5,185,108, it was not then realized that the ranges of particle sizes in each tier of the two-tier distribution controlled both, the density of the sintered part and the sinterability of the powder. Neither was it known that the distribution of particle sizes in a two-tier distribution was as critical as the viscosity characteristics of the material as a function of temperature.
The ranges of sizes in the two-tier distribution of particles used in the powder according to this aspect of the invention is different from the ranges of the two-tier distribution of the wax powder described in U.S. Pat. No. 5,185,108. Quite unexpectedly, the formation of a near-fully dense sintered part requires that at least 80% of the number of all particles in the bed are from 11 .mu.m-53.mu. and that there be substantially no (that is, &lt;5%) particles greater than 180 .mu.m (80 mesh) in a part bed. The importance of the few "large particles" to maintain (i) free-flowability near T.sub.s and (ii) a predetermined temperature profile in a part bed while a sintered part is being formed, irrespective of the density of the part formed, to negate undesirable "growth" on the part, was not then known.
Because the "selective-laser-sintering-window" may be defined by the requirements of the selective laser sintering process, the part bed (and sometimes the feed bed) is heated to near T.sub.s to negate the proclivity of the sintered layer to "curl". To minimize the curling of a slice as it lies on a part bed, it has been discovered that a preferred temperature profile is to be maintained in the bed, with a slight but narrowly specified temperature gradient on either side of a horizontal zone through the portion of the bed occupied by the sintered part, referred to as the "hot" zone.
The typical gradient in a part bed in a selective laser sintering machine is first positive, that is, the temperature increases to a maximum, then the gradient is negative, that is the temperature decreases from the maximum. The upper temperature gradient in the upper portion of the bed is positive, that is the temperature increases until it reaches a maximum temperature T.sub.max in the hot zone. The lower temperature gradient in the lower portion of the bed is negative, that is the temperature decreases from T.sub.max in the hot zone to the bottom of the bed.
More specifically, the temperature in the upper portion of the bed progressively increases as one moves downward from the upper surface of the bed to T.sub.max ; then progressively decreases as one moves downward from T.sub.max to the bottom surface of the part bed, which surface is in contact with the bed-supporting piston.
The gradient in a conventional selective laser sintering machine without controlled gas-cooling of the part bed, in each direction is typically greater than 2.degree. C./cm (5.degree. C./in). Such a gradient was found to be too high to provide an acceptable risk of distortion of the part.
These considerations lead to temperature limits in the feed and part beds which limits define the G'-window and selective-laser-sintering-window, namely, (i) the temperature at which the part bed is maintained, and the temperature profile therein, and (ii) the temperature at which the feed bed is maintained.
In turn, the temperature at which the pan bed is maintained is defined by (a) a lower (minimum) part bed temperature below which curling is so pronounced as to negate any reasonable probability of effecting a slice-wise fusion of plural vertically contiguous slices; and, (b) an upper (maximum) temperature at which interparticle viscosity in the part bed makes it so "sticky" as to fuzz (obfuscate) the predetermined boundaries of the part to be made. All sintered powder between vertically spaced apart lateral planes in the part bed is solidified sufficiently to have mechanical strength. The remaining unsintered powder remains freely-flowable.
The "improved" sinterable tailored powder provides not only the specified particle size and two-tier distribution, but also a usable and desirable selective-laser-sintering-window. The ability of a powder simultaneously to satisfy each of the requirements, provides a measure of how "good" the chance that a powder will be sinterable in the selective laser sintering process to yield a near-fully dense, but porous article.
A major practical consequence of the narrowly defined window requires that the part bed be maintained at a specified temperature and with a specified temperature profile so that each layer to be sintered lies within the confines of the selective-laser-sintering-window. A different temperature, whether higher or lower, and/or a different temperature profile, results in regions of the just-sintered initial slice of powder which will either cause a sintered slice to melt and be distorted in a layer of the part bed which has "caked"; or, will cause a sintered slice to curl if the part bed temperature is too low. In the past this has been an all too common occurrence with the result that an undesirable part was made. The tailored powder and unique bed which it forms now make production of an unacceptable part an uncommon occurrence.